1. Field of the Technology
This invention relates generally to three terminal magnetic sensors (TTMs) suitable for use in magnetic heads, which includes spin valve transistors (SVTs), magnetic tunnel transistors (MTTs), or double junction structures.
2. Description of the Related Art
Magnetoresistive (MR) sensors have typically been used as read sensors in hard disk drives. An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which causes a change in resistance of the read element and a resulting change in the sensed current or voltage.
A three terminal magnetic sensor (TTM) of a magnetic head may comprise a spin valve transistor (SVT), for example, which is a vertical spin injection device having electrons injected over a barrier layer into a free layer. The electrons undergo spin-dependent scattering, and those that are only weakly scattered retain sufficient energy to traverse a second barrier. The current over the second barrier is referred to as the magneto-current. Conventional SVTs are constructed using a traditional three-terminal framework having an “emitter-base-collector” structure of a bipolar transistor. SVTs further include a spin valve (SV) on a metallic base region, whereby the collector current is controlled by the magnetic state of the base region using spin-dependent scattering. Although the TTM may involve an SVT where both barrier layers are Schottky barriers, the TTM may alternatively incorporate a magnetic tunnel transistor (MTT) where one of the barrier layers is a Schottky barrier and the other barrier layer is a tunnel barrier, or a double junction structure where both barrier layers are tunnel barriers.
Since it is advantageous to form a very thin base region for increased areal recording densities, it has been identified that the base region in the TTM will have a relatively large electrical resistance. Given an estimated trackwidth (TW) of approximately 50 nanometers (nm) for a magnetic head, for example, the electrical resistance of the base region may be much greater than 100Ω. Thus, as the sense current passes through the base region from the emitter lead to the base lead, the base region may be prone to failure or damage (e.g. it could “blow” like a fuse). Further, a relatively large resistance for the base region raises the noise floor for the TTM, such that a much larger input signal would be required for suitable operation.
Another important consideration is that the free layer should be longitudinally biased parallel to the sensing (or ABS) plane and parallel to the major planes of the thin film layers of the TTM, such that the free layer is magnetically stabilized. This has been typically accomplished by first and second hard bias magnetic layers which are adjacent to first and second sides of the TTM. Unfortunately, the magnetic field through the free layer between the first and second sides is not uniform since a portion of the magnetization is lost in a central region of the free layer to the shields. This is especially troublesome when the track width of the TTM may be in sub-micron dimensions. End portions of the free layer which abut the hard bias layers may be over-biased and become very magnetically stiff in their response to field signals from the moving media. The stiffened end portions can take up a large portion of the total length of the TTM and can significantly reduce the signal amplitude of the TTM.
Accordingly, there is a need to solve these problems so that TTMs may be suitable for use in magnetic heads and other devices.